Flute-type wind instruments generally consist of a cylindrical tube having a central bore extending therethrough. The cylindrical tube includes a head section having a side hole mouth opening over which air is blown to produce a tone, and a main body section having a series of toneholes to play a range of notes.
The central bore extending through the head section and the side hole mouth opening have a critical effect on the quality of the tone produced by flute-type wind instruments. The primary source of the degradation of tone quality, commonly referred to as the “hissing sound,” is generated in the head section of flute-type instruments. The “hissing sound” results for the most part from the generation of turbulent flow in the central bore of the head section in the vicinity of the mouth opening.
This undesirable turbulent flow is created in part by the sharp change in the cross-sectional area of the flow path of air blown through the mouth opening and into the portion of the central bore in the head section of the instrument. The cross sectional area of the central bore in the head section of a conventional flute-type wind instrument is about four times larger than the cross sectional area of the mouth opening. The air undergoes an expansion as it flows into the central bore causing non-laminar or turbulent flow in the central bore.